The present invention relates to computed tomography (CT) imaging apparatus; and more particularly, to the backprojection of the attenuation data acquired during a scan to form an image.
In a computed tomography system, an x-ray source projects a fan-shaped beam which is collimated to lie within an X-Y plane of a Cartesian coordinate system, termed the "imaging plane." The x-ray beam passes through the object being imaged, such as a medical patient, and impinges upon an array of radiation detectors. The intensity of the transmitted radiation is dependent upon the attenuation of the x-ray beam by the object and each detector produces a separate electrical signal that is a measurement of the beam attenuation. The attenuation measurements from all the detectors are acquired separately to produce the transmission profile.
The source and detector array in a conventional CT system are rotated on a gantry within the imaging plane and around the object so that the angle at which the x-ray beam intersects the object constantly changes. A group of x-ray attenuation measurements from the detector array at a given angle is referred to as a "view" and a "scan" of the object comprises a set of views made at different angular orientations during one revolution of the x-ray source and detector. In a 2D scan, data is processed to construct an image that corresponds to a two dimensional slice taken through the imaging plane of the object.
The prevailing method for reconstructing an image from 2D data is referred to in the art as the filtered backprojection technique. The image is reconstructed from a set of views collected during one rotation. Steps for reconstruction include preprocessing, spatial filtering, and backprojection. Preprocessing includes a number of computational steps to correct the view data and to convert the data into line integral measurements. The resulting converted and corrected views are called "projections". Spatial filtering involves convolution of the projection data with a reconstruction filter kernel. Backprojection follows, and is a process of mapping the filtered projections into the image plane to create the actual image.
Preprocessing and spatial filtering involve standard mathematical operations at a processing rate often encountered in signal processing. These steps can be implemented using standard digital signal processing hardware. Backprojection requires a much higher processing rate and requires customized, dedicated hardware for efficient implementation. Backprojection of each view requires the performance of three steps for each pixel in the 2D image. First, the value of the filtered projection data must be found for the ray passing through the pixel. Usually this step requires interpolating between two adjacent filtered projection values. Second, the source-to-pixel distance is calculated, and finally, the value of the filtered projection data for the ray passing through the pixel is divided by the square of the source-to-pixel distance. These steps may require the use of three look-up tables and four separate interpolation operations to perform using current methods. As a result, image reconstruction on current CT systems is complex, costly, and a limiting factor in patient throughput.